The present invention relates generally to processes for the cryogenic distillation of air, and in particular to such processes used to produce at least a nitrogen product.
There are numerous processes which relate to the production of at least a nitrogen product by cryogenic air separation. Frequently these processes have a double column distillation unit, which utilizes a higher-pressure column and a lower-pressure column. Typically, though not exclusively, the higher-pressure column obtains a portion of its reflux through the use of a reboiler-condenser. Herein a vapor in the higher-pressure column is condensed through indirect latent heat transfer with a liquid in the lower-pressure column. Air is primarily fed to the higher-pressure column, but occasionally also may be introduced to the lower-pressure column. Meanwhile, products may be removed from either column.
Many of these processes are concerned with once through separation, wherein all fluids flow from higher pressures to lower pressures. Typically, the highest-pressure stream in once through separation cycles is a compressed air stream. These once through cycles produce products from the distillation unit at pressures no greater than that of the higher-pressure column. Post-separation compression allows for the production of products at pressures other than those found in the cryogenic air separation process. Two such embodiments of post-separation compression are found in pumped LOX cycles and pumped LIN cycles. In these embodiments a liquid product is removed from the distillation unit, pumped to an elevated pressure, and delivered to a warm elevated pressure product. In JP 1062062, U.S. Pat. No. 5,906,113 (Lynch et al.), U.S. Pat. No. 4,582,518 (Erickson) and U.S. Pat. No. 5,918,482 (Potempa), liquid nitrogen is removed from a single column cycle and provides a portion of the reflux for an additional column.
A second set of cycles exist wherein a liquid stream is removed from the lower-pressure column and its pressure increased, for example through pumping. This elevated pressure stream is eventually returned to the cryogenic air separation cycle. These cycles may be described as pump-back cycles, and do not pertain to the set of once through cycles.
An advantage of a pump-back cycle is that products may be produced at pressures greater than that of the lower-pressure column. These cycles are especially beneficial if a single, high pressure product is required. However, the pressure of the product stream is still bounded by the pressures found in the higher and lower-pressure columns.
In U.S. Pat. No. 5,964,104 (Rottmann) liquid nitrogen from the lower-pressure column of a double column cycle is pumped to the higher-pressure column where it is used as reflux. In WO 98/19122 (Corduan) liquid nitrogen from the lower-pressure column of a double column cycle is pumped to a heat exchanger where it is either fully or partially vaporized. A portion of the reflux for the higher-pressure column is provided by indirect condensation with this boiling pumped liquid nitrogen. These cycles just described do not contain additional columns.
A third set of cycles exists where an additional or supplemental column is used. These supplemental columns are known in the prior art as Intermediate Pressure Columns (IP), or Medium Pressure Columns (MP). Most of these cycles improve the once through cycles by removing a product at a pressure between that of the higher-pressure column and the lower-pressure column. A typical method of operation is when a stream of liquid is removed from the higher-pressure column to reflux the supplemental column. This removal of higher-pressure reflux tends to reduce the production of nitrogen product from the higher-pressure column. The pressure of the nitrogen product from this supplemental column remains bounded between the higher and lower-pressure columns.
In U.S. Pat. No. 5,069,699 (Agrawal) air is sent to the higher-pressure column of a double column cycle and an extra high pressure (EHP) column. A gaseous nitrogen stream from the EHP column is condensed indirectly with an oxygen enriched liquid in the lower-pressure column. A portion of this condensed EHP gaseous nitrogen stream is used as reflux for the high-pressure (HP) column. In EP 0921367 and EP 0924486 liquid nitrogen produced in the third column, at a pressure typically around 90 psia, may be used as reflux for both the higher-pressure and lower-pressure columns. In all of these cycles no portion of the nitrogen product is removed from the additional column.
In U.S. Pat. No. 3,688,513 (Streich, et al.) liquid nitrogen from the lower-pressure column of a double column cycle is pumped and used as a portion of the reflux for the IP column. A product of enriched oxygen is removed from the bottom of the IP column. In this cycle an oxygen enriched liquid from the bottom of the IP column is not sent to the distillation unit.
In U.S. Pat. No. 4,533,375 (Erickson) and U.S. Pat. No. 4,605,427 (Erickson) the lower-pressure column is refluxed through the vaporization of liquid nitrogen. In these cases just described, the vaporizing liquid has an oxygen concentration less than that of air.
In U.S. Pat. No. 5,730,004 (Voit) and U.S. Pat. No. 4,254,629 (Olszewski) air is sent to an IP column. Reflux for this IP column is provided by condensing nitrogen indirectly against a boiling oxygen enriched liquid. A portion of the condensed IP nitrogen liquid is used as reflux to the lower-pressure column of a double column cycle. In U.S. Pat. No. 5,485,729 (Higginbotham) an IP column derives reflux by condensing gaseous nitrogen in intermediate reboiler condensers located within a lower-pressure column of a double column cycle. A portion of the liquid nitrogen produced is used to reflux the lower-pressure column. In U.S. Pat. No. 5,402,647 (Bonaquist, et al.) a third column, operating at a pressure generally between 30 psia and 60 psia, produces a liquid nitrogen product, which is pumped to the higher-pressure column where it is used as reflux. In these cases, no liquid from the distillation unit is raised in pressure and sent to either the additional column or returned to the distillation unit.
In EP 1043558 (Brugerolle) liquid nitrogen is pumped from a distillation unit to a power producing cycle. Herein, an oxygen-enriched fluid is recovered and returned to the distillation unit. The nitrogen-enriched gas produced from the top of the column is injected into the gas turbine ensuring that the mass flowrate to the expander is not compromised. This reference describes the increase of production of oxygen from the distillation unit and also describes cycles known in the prior art as oxygen plants. A liquid stream is therefore not removed from the lower-pressure column and vaporized in such a manner that a reflux stream is produced.
It is desired to have an improved air separation process for the production of nitrogen.
It is further desired to have an improved air separation process for the production of nitrogen which overcomes the difficulties and disadvantages of the prior art processes to provide better and more advantageous results.
A first embodiment of the invention is a process for separating a multi-component fluid comprising oxygen and nitrogen to produce nitrogen. The process uses a distillation column system having at least three distillation columns, including a higher-pressure column operating at a first pressure, a lower-pressure column operating at a second pressure lower than the first pressure, and a supplemental column operating at a third pressure greater than or equal to the second pressure. The higher-pressure column and the lower-pressure column are thermally linked through a first heat exchanger. Each distillation column has a top, a bottom, and a plurality of locations between the top and the bottom. The process includes multiple steps. The first step is to feed a first stream of the multi-component fluid to the higher-pressure column. The second step is to feed a second stream of the multi-component fluid or another multi-component fluid comprising oxygen and nitrogen to the supplemental column. The third step is to withdraw a first nitrogen-rich vapor stream from the higher-pressure column or the lower-pressure column. The fourth step is to withdraw a first oxygen-rich liquid stream from the lower-pressure column. The fifth step is to heat exchange at least a portion of the first oxygen-rich liquid stream indirectly against at least a portion of the first nitrogen-rich vapor stream in the first heat exchanger or a second heat exchanger, thereby at least partially vaporizing the first oxygen-rich liquid stream and at least partially condensing the first nitrogen-rich vapor stream. The sixth step is to eventually change the pressure of at least a portion of the condensed first nitrogen-rich vapor stream. The seventh step is to eventually feed at least a portion of the condensed first nitrogen-rich vapor stream to the supplemental column. The eighth step is to withdraw a second oxygen-rich liquid stream from the supplemental column. The ninth step is to feed at least a portion of the second-oxygen-rich liquid stream to the lower-pressure column or the higher-pressure column. The tenth step is to withdraw a first stream of nitrogen product from the supplemental column.
There are several alternate embodiments of the invention. One alternate embodiment is similar to the first embodiment but includes the additional step of withdrawing a stream of a product enriched in oxygen from the lower-pressure column. Another alternate embodiment is similar to the first embodiment but includes the additional step of withdrawing a stream of product enriched in nitrogen from the higher-pressure column.
There also are many variations of the first embodiment. In one variation, the third pressure is greater than or equal to the first pressure. In another variation, a first nitrogen-rich liquid stream from the first heat exchanger is fed to the lower-pressure column at a first location, and a second nitrogen-rich liquid stream from the second heat exchanger is fed to the lower-pressure column at a second location above the first location.
In another variation of the first embodiment, the pressure of the portion of the condensed nitrogen-rich vapor stream is changed by reducing the pressure. A variant of this variation includes several additional steps. The first additional step is to withdraw a second nitrogen-rich vapor stream from the lower-pressure column. The second additional step is to withdraw a third oxygen-rich liquid stream from the lower-pressure column. The third additional step is to heat exchange at least a portion of the third oxygen-rich liquid stream indirectly against at least a portion of the second nitrogen-rich vapor stream in a second heat exchanger, thereby at least partially condensing the second nitrogen-rich vapor stream. The fourth additional step is to increase the pressure of at least a portion of the condensed nitrogen-rich vapor stream. The fifth additional step is to feed at least a portion of the condensed second nitrogen-rich vapor stream to the higher-pressure column.
In another variation of the first embodiment, the pressure of the portion of the condensed first nitrogen-rich vapor stream is changed by increasing the pressure. There are several variants of this variation. One variant includes several additional steps. The first additional step is to withdraw a second nitrogen-rich vapor stream from the supplemental column. The second additional step is to withdraw a third oxygen-rich liquid stream from the lower-pressure column. The third additional step is to heat exchange at least a portion of the third oxygen-rich liquid stream indirectly against at least a portion of the second nitrogen-rich vapor stream and a third heat exchanger, thereby at least partially condensing the second nitrogen-rich vapor stream. The fourth additional step is to feed at least a portion of the condensed second nitrogen-rich vapor stream to the supplemental column. In a variation of this variant, a portion of the condensed first nitrogen-rich vapor stream is fed to the supplemental column at a first location, and a portion of the condensed second nitrogen-vapor stream is fed to the supplemental column at a first location or at a second location above the first location.
Another aspect of the present invention is a cryogenic air separation unit using a process as in any of the embodiments, variations, or variants of the process discussed herein.